1. Field of the Invention
The present invention relates generally to an image forming optical system and more particularly to means for improving the image forming characteristics with a unique filter structure and mounting.
2. Description of the Prior Art
The desirability of improving the imaging characteristics of various lens systems are well known in the prior art. For example, it is known to use a soft focus filter to produce a faithful but soft tone portrait in the field of photography. Frequently, optical elements are permanently mounted in the lens barrel, traversely removable from the lens barrel or attached to the end of the lens barrel.
Also, it is highly desirable to improve the images that are not conjugate to the image or film plane. For example, the background objects behind a principal target object must supplement or complement the photograph of the principal image while still retaining a general outline that is softly blended.
An image can be defined by a spatial distribution of illumination existing in a determined plane which is the plane of the image. It has been known to determine the image properties of an optical system not only by the phase of the pupil function but also by its amplitude transmittance. For any optical system, a family of curves may be established which express the transmission factor as a function of the spatial frequency or wavelength. By the use of Fourier transformation the imaging properties of an optical system can be measured by the optical transfer function. Any optical system and therefore any optical instrument, may thus be considered as a transmitter of signals corresponding to sinusoidal functions in which the function is the luminance of a point and the variable is the position of the point. It is known that by decreasing the transmitted image point intensity of a defocussed image the photographic characteristics of the image can be improved. This improvement can be realized in other optical systems than cameras, such as microscopes and telescopes.
The prior art has suggested various ways of improving the optical transfer function of a defocussed optical system such as the Mino et al U.S. Pat. No. 3,843,235 wherein a filter is provided having a characteristic of providing an optical transfer function of a monotonously decreasing function in accordance with an increase in spatial frequency. Further the optical transfer function must remain essentially positive.
Various optical systems have been suggested for providing a filter to provide a desired image such as the Tsujiuchi U.S. Pat. No. 3,045,530, Marechal et al U.S. Pat. No. 3,090,281, Sayanagi U.S. Pat. No. 2,959,105, Nomarski U.S. Pat. No. 3,476,457, and Land U.S. Pat. No. 3,397,023.
As noted above, a photographic lens assembly is generally evaluated in accordance with the sharpness of a focussed image on a film plane. However, the commercial acceptability of a photographic picture frequently requires images of photographic objects located both frontwards and rearwards of a sharply defined target photographic object to have a pleasing natural appearing blurred image. In some cases it is even desired to provide a blur or soft defocussed image for the target photographic object itself. The Mino et al patent mentioned above, is representative of one such apodization optical system.
The combining however, of such a filter element in a commercial lens system has caused problems, for example, in focussing and setting the diaphragm aperture. Generally, the optical filters have been directly attached to the exterior lens or lens barrel or affixed at one position in the interlens space of the optical system. The prior art has not provided a versatile commercially acceptable lens in a compact mode which permits a selective utilization of a filter optical element.